Flat panel displays are widely used in a variety of applications, including computer displays. One suitable flat panel display is a field emission display. Field emission displays typically include a generally planar emitter substrate covered by a display screen. A surface of the emitter substrate has formed thereon an array of surface discontinuities or "emitters" projecting toward the display screen. In many cases, the emitters are conical projections integral to the substrate. Typically, contiguous groups of emitters are grouped into emitter sets in which the bases of emitters in each emitter set are commonly connected.
The emitter sets are typically arranged in an array of rows and columns, and a conductive extraction grid is positioned above the emitter. All, or a portion, of the extraction grid is driven with a voltage of about 30-120 V. Each emitter set is then selectively activated by applying a voltage to the emitter sets. The voltage differential between the extraction grid and the emitter set produces an electric field extending from the extraction grid to the emitter set having a sufficient intensity to cause the emitters to emit electrons.
The display screen is mounted directly above the extraction grid. The display screen is formed from a glass panel coated with a transparent conductive material that forms an anode biased to about 1-2 kV. The anode attracts the emitted electrons, causing the electrons to pass through the extraction grid. A cathodoluminescent layer covers a surface of the anode facing the extraction grid so that the electrons strike the cathodoluminescent layer as they travel toward the 1-2 kV potential of the anode. The electrons strike the cathodoluminescent layer, causing the cathodoluminescent layer to emit light at the impact site. Emitted light then passes through the anode and the glass panel where it is visible to a viewer. The light emitted from each of the areas thus becomes all or part of a picture element or "pixel."
The brightness of the light produced in response to the emitted electrons depends, in part, upon the rate at which electrons strike the cathodoluminescent layer. The light intensity of each pixel can thus be controlled by controlling the current available to the corresponding emitter set. To allow individual control of each of the pixels, the electric potential between each emitter set and the extraction grid is selectively controlled by a row signal and a column signal through corresponding drive circuitry. To create an image, the drive circuitry separately establishes current to each of the emitter sets.
In some embodiments, the voltage difference between the extraction grid and the emitter sets is controlled by setting the entire extraction grid to a single voltage and selectively coupling each emitter set to a reference potential, such as ground. One drawback of such an approach is that the drive circuitry for each of the emitter sets must respond to both a column signal and a row signal. This approach typically requires separate transistors or other current control elements for each of the column signal and the row signal such that each pixel requires at least a pair of current control elements.
Another approach to controlling the voltage differential between the extraction grid and the emitter sets is to divide the extraction grid into discrete sections, where each section corresponds to a column of an array. Then, the array of emitter sets is divided into rows where emitter sets in each row are tied to a common row line.
To activate this structure, one of the row lines is first grounded. Then, each of the column lines in the extraction grid is driven by a voltage corresponding to an image signal. To produce bright pixels, the column lines of the extraction grid are raised to a high voltage and to produce dim pixels, the column lines are held at a low voltage. The column lines are therefore driven by rapidly switching, high analog voltages that require relatively expensive driver circuitry.
Another approach to activating the display is to drive the sections of the extraction grid with a constant magnitude voltage in response to the row signal and to drive columns of the emitter substrate with analog voltages corresponding to the image signal. Sections of the extraction grid are thus the row lines and lines of the emitter substrate are the column lines. In this approach, the row lines of the extraction grid are selectively biased at a constant grid voltage V.sub.G, one row at a time. During the time a row line of the extraction grid is biased, each column line of the emitter substrate receives an analog column voltage corresponding to an image signal. The emitter set in the column that intersects the biased column of the extraction grid will therefore emit light when the column line voltage is sufficiently below the voltage of the biased extraction grid row. The intensity of the emitted light will depend upon the voltage of the column line. If the column line voltage is very far below the grid voltage V.sub.G, the pixel will be bright. If the column voltage is not very far below the grid voltage V.sub.G, the pixel will be dim.
In this approach, the time during which each of the columns of the emitter substrate is active is only a small part of the time during which each row of the extraction grid is activated. Consequently, only a brief "window" is available to drive each of the column lines.
Because only a brief window is available, the column line must be pulled quickly down to the appropriate voltage. However, the electrical characteristics of the column line, such as its resistance and capacitance, can limit the speed at which the column line voltage can change. For example, the column line includes a distributed capacitance. Therefore, resistance between a signal input and the column line combines with the distributed capacitance to form an RC circuit whose time constant limits the speed at which a voltage applied to the column line can be coupled to the emitter sets in that column. Consequently, a brief input pulse at one end of the column line may not establish the emitter sets in the column line at the appropriate voltage. The duration of the input pulse is not easily increased, because the length of the pulse is limited by the window described above. The available pulse time can be lengthened somewhat by extending the refresh time of the pixels (i.e., the time between successive activations of an emitter set), because extending the refresh time increases the size of the window. However, this approach correspondingly reduces the rate at which an image can be "written," thereby impairing the operation of the display.